What is NROL-47 and in what orbit will it be launched? [updated]

UPDATE 10 Jan 17:25 UT: The launch has been scrubbed due to high altitude winds, and moved one day to Jan 11. New start of launch window is given as 1 pm PST = 20:00 21:00 UT. This means the launch window is shifting, indicating a prefered orbital plane and launch probably right at the start of the launch window.

Hot after the excitement and drama of the Zuma launch (see my previous post), a new classified launch is upcoming on Wednesday January 10, when ULA will launch NROL-47, a classified payload for the National Reconnaissance Office (NRO), on a Delta IV from Vandenberg SLC-6 in California.

From Maritime Broadcast Warnings, the launch window opens at 20:30 UT and closes at 01:26 UT.

The launch is in Westward direction, into retrograde orbit. This has led some space news websites to assume that the NROL-47 payload is the 5th TOPAZ (FIA Radar) satellite.

But is it? I have some doubts.

If it is TOPAZ 5, then it is clearly a deviation from the previous four launches. The launch hazard zones from published Maritime Broadcast Warnings show that the launch azimuth is different – previous TOPAZ missions all launched into azimuth 220 degrees, but NROL-47 launches into azimuth 200 degrees, a 20 degree difference.

NROL-47 Launch hazard areas (red) compared to the areas of four TOPAZ (FIA Radar) launches
click map to enlarge

This can be clearly seen on the map above, where the NROL-47 hazard zones are in red, and the hazard zones from the four TOPAZ in purple, green, light blue and dark blue. The azimuth and locations of the zones from the four TOPAZ launches are all quite similar, but those of NROL-47 stand out as different.

All the four TOPAZ satellites are in a 123.0 degree inclined retrograde orbit. The NROL-47 launch azimuth results in a retrograde orbit too, but with an orbital inclination of 108.6 degrees, not 123.0 degrees: a 14.4 degree difference.

The orbital altitude aimed for appears to be different too. The four TOPAZ satellites are in 1100 x 1110 km orbits. But the location of the Delta IV Upper Stage de-orbit zone (between South Africa and Antarctica), its shape and the opening time of the window (23:23 UT) points to the NROL-47 payload going into a 1500 km altitude orbit instead.

estimated trajectory of NROL-47
click map to enlarge

So if this is the 5th TOPAZ launching as NROL-47 on Wednesday, then it is going into a quite different orbit compared to the previous four TOPAZ: different in orbital inclination as well as in orbital altitude.

In theory, the Delta IV rocket could do a “dogleg” and (when launching at 20:30 UT) deliver the NROL-47 payload into the 123.0 degree inclined orbit close to the orbital plane of TOPAZ 1 (FIA Radar 1). A second manoeuvre near the south polar pass could then align the RAAN and bring it exactly into the orbital plane of TOPAZ 1.

But why do that, if previous TOPAZ launches simply launched directly into the 123.0 degree inclination orbit?

So in my view, the jury is still out regarding the identity of NROL-47. It could be a 5th TOPAZ but in a quite different orbit compared to the previous four (in itself possible: the Lacrosses also occupied two different orbital inclinations). It could also be something new. If something new, it likely will be a radar satellite (like TOPAZ), given the retrograde character of the orbit.

orbital constellation of TOPAZ 1, 2, 3 and 4 in 123.0 degree inclined orbits
The orbits are spaced 90 degrees in RAAN
click image to enlarge

The deliberate re-entry of the Upper Stage happens 1.5 revolutions (2h 55m) after launch.

Estimated search orbits, based on a 108.6 degree orbital inclination, are here. South Africa will have two visible passes after launch.Source sattrackcam.blogspot.com